1. Field of the Invention
This invention relates to a circuit structure of a liquid crystal display device, and more particularly to a pixel unit driving circuit, a driving method and a display device.
2. Description of the Prior Art
An Organic Light Emitting Diode (OLED) is increasingly applied in a high performance display device as a current mode light emitting device. Since a conventional passive matrix OLED needs shorter driving time for an individual pixel as the display size is increased, a large transient current and a large power consumption are required. Meanwhile, applying a large current will result in a too large voltage drop on an Indium Tin Oxide (ITO) line, and the operation voltage of OLED is too high, its efficiency is decreased. An Active Matrix OLED (AMOLED) progressively scans inputted OLED current through a switch transistor so that above mentioned problems are solved.
In the design of an AMOLED back panel, the main problem needs to be solved is that brightness between pixels is not uniform.
Firstly, AMOLED builds up a pixel circuit by using a Thin Film Transistor (TFT) so as to provide corresponding current for an OLED device, for example by using a low temperature poly-silicon thin film transistor (LTPS TFT) or an oxide TFT. As compared with a general Amorphous Silicon TFT (a-Si TFT), LTPS TFT and Oxide TFT have the characteristics of having a higher migration rate and being more stable, which are more suitable for AMOLED display. However, since a crystallization process has its limitations, LTPS TFT manufactured in a large area of a glass substrate has nonuniform electrical characteristics, such as a threshold voltage and a migration rate. Such nonuniformity will result in current difference and brightness difference of an OLED display device which will be sensed by human eyes, that is a mura phenomena. Although the process of Oxide TFT has a good uniformity, like a-Si TFT, its threshold voltage will shift under pressure and high temperature for a long time. Since display pictures are different, the amounts of shift on the threshold values of TFTs on each part of the panel are different, which will result in differences in display brightness. Since such difference is related to the pictures that shown previously, a ghost phenomena will exhibit.
Secondly, in an application of displaying in a large size, since the power line of the back panel has a certain resistance, the power voltage close to ARVDD power supply position is higher than the power voltage away from the power supply position, which is called IR Drop. IR Drop will result in differences in current in different areas which leads to the mura is generated during the displaying.
AMOLED may be divided into three modes based on driving types: a digital mode, a current mode and a voltage mode. The driving in the voltage mode has the advantages of a fast driving speed and a simple implementation. The voltage mode is suitable for driving a large size panel, but additional TFTs and capacitors are required to compensate the nonuniformity of TFTs, the nonuniformity of IR Drop and OLED.
FIG. 1 is a traditional voltage driving mode pixel circuit structure composed of two TFTs and one capacitor (2T1C). Wherein, a switch transistor T2 transmits the voltage in a data line to the gate electrode of a driving transistor T1, the driving transistor T1 transfers the data voltage into the corresponding current to provide power to the OLED device. During a normal operation, the driving transistor T1 is in the saturation region and provides a constant current during the time for scanning one line. The current can be represented as:
      I    OLED    =            1      2        ⁢                  μ        n            ·      Cox      ·              W        L            ·                        (                      Vdata            -            Voled            -            Vth                    )                2            
Wherein, μn is a carrier mobility, COX is a gate oxide capacitance, W/L is a width to length ratio of the transistor, Vdata is the voltage in the data line, Voled is the operating voltage of OLED shared by all pixel units, Vth is the threshold voltage of the transistor, for an enhanced TFT, Vth is a positive value and for a depleted TFT, Vth is a negative value. As shown in the above equation, if different pixel units have different Vths, their currents are different. If Vth of a pixel is shifted over time, the current may changed over time and ghosting may be caused. Since the nonuniformity of the OLED devices causes that work voltages of OLEDs are different, which also causes current differences.
There are many kinds of pixel structures which aim to compensate the nonuniformity of Vth and the shift, and nonuniformity of OLED. Such structures are commonly implemented by connecting TFTs by diodes as shown in FIGS. 2-3. However, such structures are only suitable for the enhanced TFTs, the voltage stored in a depleted TFT does not have the voltage information of Vth which leads to the nonuniformity of Vth cannot be compensated.